Biology by Bradford
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The subtopic in the middle and eleven tasks round it: label five diagrams, complete a table, put a process in order and fill 57 gaps. Every standard level statement has a box; the four boxes for higher level only are dashed and badged HL, and drop off for an SL class. Type into the blanks and check them, or print it on A4, Letter or A3 with an answer key.
Short carbohydrate chains are attached to some membrane proteins, making , and to some lipids, making . The chains are only ever on the side of the plasma membrane. Their shapes act as markers in cell , so the immune system can tell self from non-self, and they help neighbouring cells stick together in cell .
| Route across | Concentration gradient | |
|---|---|---|
| Simple diffusion | between the phospholipids | |
| Facilitated diffusion | through | |
| Active transport | through proteins that use energy |
Channels and pumps each pass only particular particles, so they make a membrane selective. Simple diffusion is not: which particles cross depends only on their and on whether they are hydrophilic or hydrophobic. O₂ and CO₂ cross this way, being small and .
Kinked fatty acid tails pack loosely and have lower melting points, keeping a membrane fluid and flexible at the temperatures a cell meets. Straight tails pack tightly and make a membrane stronger at higher temperatures; fish of polar seas have more of the kinked kind in their membranes. In animal cells, sits among the phospholipids and adjusts fluidity: it the membrane when warm and stops it becoming when cold.
Number the steps in order.
Each turn moves more positive charge out than in, helping to make the inside of a neuron compared with the outside: a membrane potential. Na⁺ and K⁺ channels in neurons open when that potential changes, so they are -gated; nicotinic receptors open when binds, so they are neurotransmitter-gated.
Both rely on the membrane being fluid, so that it can pinch apart and join up again without leaking. A takes in bacteria the way shown on the left; a pancreas cell releases insulin the way shown on the right.
1234Na⁺ is kept at a concentration inside the cell, so it diffuses in from the gut, and glucose is carried in with it its own concentration gradient. ATP is not used at this step, so this is active transport. Cells of the nephron use the same method to glucose from the filtrate.
Phospholipids are : each has a hydrophilic phosphate head and two hydrophobic tails. In water they form a continuous , heads facing the water on each side and tails meeting in the core. That core lets very few large molecules, or polar molecules through, so the membrane is an effective between two watery solutions.
Fluid: the lipids and most proteins drift within their own layer. Mosaic: proteins of many kinds lie scattered among the lipids, some carrying particles across, some acting as for signals and some as .
Water molecules move at and cross the membrane in both directions, many of them through protein pores called . The membrane is to the solutes, so they cannot follow, and there is a net movement of water towards the side with the solute concentration: osmosis.
The left-hand protein has a pore that fits only one kind of ion, and ions diffuse through only while its gate is , so the membrane is permeable. The right-hand protein uses energy from the small molecule beside it to carry particles their concentration gradient.
Phospholipids are amphipathic: each has a hydrophilic phosphate head and two hydrophobic hydrocarbon tails. In water they form a continuous bilayer, heads facing the water on each side and tails meeting in the core. That core lets very few large molecules, ions or polar molecules through, so the membrane is an effective barrier between two watery solutions.
Fluid: the lipids and most proteins drift sideways within their own layer. Mosaic: proteins of many kinds lie scattered among the lipids, some carrying particles across, some acting as receptors for signals and some as enzymes.
Short carbohydrate chains are attached to some membrane proteins, making glycoproteins, and to some lipids, making glycolipids. The chains are only ever on the extracellular side of the plasma membrane. Their shapes act as markers in cell recognition, so the immune system can tell self from non-self, and they help neighbouring cells stick together in cell adhesion.
Water molecules move at random and cross the membrane in both directions, many of them through protein pores called aquaporins. The membrane is impermeable to the solutes, so they cannot follow, and there is a net movement of water towards the side with the higher solute concentration: osmosis.
The left-hand protein has a pore that fits only one kind of ion, and ions diffuse through only while its gate is open, so the membrane is selectively permeable. The right-hand protein uses energy from the small molecule beside it to carry particles against their concentration gradient.
| Route across | Concentration gradient | |
|---|---|---|
| Simple diffusion | between the phospholipids | down it |
| Facilitated diffusion | through channel proteins | down it |
| Active transport | through proteins that use energy | against it |
Channels and pumps each pass only particular particles, so they make a membrane selective. Simple diffusion is not: which particles cross depends only on their size and on whether they are hydrophilic or hydrophobic. O₂ and CO₂ cross this way, being small and non-polar.
Kinked unsaturated fatty acid tails pack loosely and have lower melting points, keeping a membrane fluid and flexible at the temperatures a cell meets. Straight saturated tails pack tightly and make a membrane stronger at higher temperatures; fish of polar seas have more of the kinked kind in their membranes. In animal cells, cholesterol sits among the phospholipids and adjusts fluidity: it stabilises the membrane when warm and stops it becoming stiff when cold.
Both rely on the membrane being fluid, so that it can pinch apart and join up again without leaking. A white blood cell takes in bacteria the way shown on the left; a pancreas cell releases insulin the way shown on the right.
Each turn moves more positive charge out than in, helping to make the inside of a neuron negative compared with the outside: a membrane potential. Na⁺ and K⁺ channels in neurons open when that potential changes, so they are voltage-gated; nicotinic receptors open when acetylcholine binds, so they are neurotransmitter-gated.
Na⁺ is kept at a low concentration inside the cell, so it diffuses in from the gut, and glucose is carried in with it against its own concentration gradient. ATP is not used at this step, so this is indirect active transport. Cells of the nephron use the same method to reabsorb glucose from the filtrate.